The present invention relates in general to a superconducting linear particle accelerator which is loaded with a sapphire dielectric.
There is currently a need to design a linear accelerator (linac) suitable for a TeV e.sup.+ /e.sup.- linear collider. This energy level requires that a conventional copper linac have an energy source capable of producing rf peak power levels on the order of 100 MW/meter. The need for such a high rf peak power presents difficult practical problems. This concept is pursued nevertheless because it is believed to be a way to achieve the high accelerating gradient needed to provide TeV energies within reasonable lengths (on the order of 10 km). If it were possible to make superconducting linacs with comparable gradients, it would be preferable to do so, since the demands on peak rf power would be significantly less. At present, however, state-of-the-art superconducting linacs have gradients only on the order of 5 MV/m, although gradients as high as 20 MV/m with Nb cavities have been produced under carefully controlled laboratory conditions. It is believed that the ultimate limit of such cavities may be as high as 30 MV/m, although the cost to manufacture such an accelerator would be prohibitive. A superconducting linac would be much longer than a conventional copper linac, since the gradients achieved so far are about ten times lower than for copper linacs. The advantage of low peak power is traded against the disadvantage of greater length.
Conventional copper linacs employ irises to slow down the phase velocity of the accelerating wave. These irises are spaced along the length of the linac, and must be manufactured and positioned with extreme precision to avoid problems with wakefields that are generated by charged particles (e.g. electrons) as they are accelerated through the irises. An alternative approach is to load a cylindrical waveguide with dielectric material rather than with irises. This is advantageous in its simplicity of construction. Unfortunately, loss tangents of typical dielectric materials are several times 10.sup.-4 at best, so there is significant rf heating in the dielectric, in addition to the skin effect ohmic losses in the conductor. It is also possible that rf breakdown could be worse for the dielectric linac because the electric field is along the dielectric surface. As a result, prior dielectric linac structures would not be suitable for the high energy requirements of a 1 TeV linear collider. What is needed is a linac structure that permits the simpler structure of a dielectric linac in a superconducting environment.